Obesity is a problem of energy balance. When energy expenditure does not keep up with intake, then adipose tissue accumulates to excess levels. Modern tools are available for monitoring two of the three components of the energy equation: body composition and energy expenditure. Energy intake, however, is still evaluated by standard self-report questionnaires on food intake. The questionnaire-based tools are useful in providing data on dietary patterns, but can be inaccurate and biased especially for overweight individuals who may underreport their energy intake. The purpose of the proposed exploratory research is to develop a practical tool that can measure average energy intake in free-living adults. In this proposal we present evidence that simple monitoring of isotope clearance in breath CO2 can provide quantitative information on fat, protein and carbohydrate intake. Our approach includes both mathematical modeling and clinical validation. The mathematical model will simulate the kinetics of C-13 labeled fatty acid as a function of fat, protein and carbohydrate intake, body composition, and energy expenditure. A dose of C-13 labeled fatty acid is followed by C-13 detection in breath for several days. Work with a preliminary computer simulation model indicates that fat and protein intake have different and predictable effects on the shape of the C-13 elimination curve. The detailed kinetic parameters of the isotope clearance will be measured experimentally. A second experiment will test the hypothesis that the C-13/C-12 ratio in breath after dosing with C-13 labeled fatty acid is an index of total fuel intake, by using a total body carbon pool model and dosing with every meal. The outcome of this research will be a fully developed stable-isotope based tool to serve energy balance studies by evaluating energy intake. If successful, it will become for energy intake what doubly labeled water is today for energy expenditure.